1. Field of the Invention
The present invention generally relates to a preheat control device for modulating the voltage of a gas-discharge lamp and, more particularly, to a preheat control device using pulse-width modulation (PWM) for controlling the voltage of the gas-discharge lamp while it is operating at a preheat state so as to effectively reduce the voltage of the gas-discharge lamp and avoid the glow current while maintaining the preheat current and using pulse frequency modulation (PFM) for controlling the frequency of the gas-discharge lamp while it is operating at a steady state so as to stabilize the current flowing through the lamp. The method of the present invention can be implemented using an analog integrated circuit (IC) or a digital controller without modifying the currently used half-bridge driver and resonant tank network.
2. Description of the Prior Art
The currently used electronic ballasts use half-bridge drivers with half-bridge configurations. In order to be compatible with the design of integrated circuits (ICs), the half-bridge configurations are implemented using class-D design, which exhibits standard half-bridge characteristics and system grounding. Therefore, the class-D design is advantageous in that only one DC-link capacitor is required at the input terminal so as to effectively reduce the manufacturing cost of the electronic ballasts. Please refer to FIG. 1, which is a diagram showing the voltage and the current of a conventional electronic ballast while it is operating at a preheat state, a firing state and a steady state. The conventional electronic ballasts are controlled by PFM during the preheat, firing and steady states. In FIG. 1, when the electronic ballasts using class-D design is at a preheat state, the lamp voltage VLAMP is half of the DC-link voltage because the load is empty-loaded. The filament current IF generates a glow current at the firing state. If the lamp is frequently turned on and off, the lifetime of the lamp will be shortened. Therefore, there is need in providing a method for overcome the problems due to the lamp voltage during the preheat state.
Generally, there are two solutions for the above-mentioned issue: one is the use of the standard half-bridge configuration and the other is the use of an additional external switch to set the lamp voltage to be zero. However, these two solutions bring forth some drawbacks. For example, the DC-link capacitor is increased for the former and the cost is higher for the latter because of the additional circuitry and the external switch. These drawbacks weaken the marketing competitiveness of the conventional electronic ballasts. Therefore, the currently used configuration can be used and modified to overcome the foregoing problems with shortened time-to-market.